Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. Considerable efforts are being made to develop wind turbines that are reliable and efficient. Generally, a wind turbine includes a rotor with multiple wind turbine blades. The wind turbine blades are shaped as elongated airfoils configured to provide rotational forces in response to wind. One current approach to manufacturing wind turbine blades is to produce each blade either as two half shells and a spar, or as two half shells with an integral spar. In both cases, the two half shells are bonded together along their edges with an adhesive material to form the complete blade. Typically, the adhesive material is a two-component (2K) structural adhesive material that includes two components that chemically react (i.e., crosslink) when mixed under ambient conditions to bond together the half shells.
The adhesives that are utilized to couple the wind turbine blade halves must be able to withstand the centrifugal forces applied to each blade during use and maintain bond strength for the blade's lifetime under constant thermal cycling and environmental attack. In addition, these adhesive materials should be relatively easy to apply. Thus, for 2K adhesives, pot life is an important consideration. The term “pot life”, as those of ordinary skill in the adhesives arts recognizes, may be defined as the time period in which the adhesive composition is sufficiently liquid such that it may be applied to a substrate material to be bonded. An adhesive material with a shorter pot life is wherein the two components react more quickly, and an adhesive material with longer pot life is wherein the two components react more slowly.
Another aspect to consider, in particular as demand grows for longer blades, is open time. The term “open time” may be defined as the time period between the application of the adhesive on one half shell and the placement of the second half shell on the adhesive bond line. An adhesive material providing good adhesive properties even after a long open time is preferred. Another aspect to consider is the environmental conditions, temperature and humidity, under which the half shells are bonded together. Ideally, an adhesive material is robust over a wide range of conditions, and not sensitive to adverse environmental conditions.
Wind turbine blade manufacturers are being challenged because of demands for larger blades. In the past, the largest operating wind turbines had rotor diameters of less than 100 meters. The rotor diameter is approximately twice the blade length, so the largest blades on these rotors were less than 50 meters. Presently, some onshore wind turbine rotor diameters are as large as 130 meters and some offshore turbines have rotor diameters of 170 meters. However, as the blades become longer, the currently available adhesives cause processing problems.
The use of epoxy-based adhesives to bond wind turbine blade shells is well-known. Typical related art can be found, for example, in DE102011113395 and US2012128499. Prior art adhesives do not have sufficiently long pot life, are sensitive to long open times and/or do not have the right reactivity at elevated temperatures to meet the demands casued by the increasing blade sizes.
FIG. 1 shows tensile data generated using a standard wind blade adhesive reference epoxy curing agent with a diglycidyl ether of bisphenol A (DGEBA)-type liquid epoxy resin, having an epoxy equivalent weight (EEW) of 190. Under normal conditions (no open time), the force required to pull two substrates was 8 MPa at an elongation of about 1%. Using the same curing agent and substrates, but with an open time of 1 hour at 23° C. and a relative humidity of 70%, the force required to pull the substrates dropped to 2 MPa (75% decrease) at an elongation of only 0.3%.
Due to the deficiencies discussed above, blade manufacturers have been faced with blade failures, such that the adhesive between the two shells fails. Thus, there is a need in the art for adhesive compositions that have a long pot life, good bonding capability after a long open time, and effective under varied environmental conditions.